In the following, reference will often be made to the magnitude of an underpressure in the tank-venting apparatus. In this context a "higher" underpressure means an underpressure of a high amount. Correspondingly, a threshold underpressure is exceeded by an actual underpressure when the amount of the actual underpressure is greater than the amount of the threshold underpressure.
An arrangement for conducting a tank-venting diagnosis is described in U.S. Pat. No. 4,962,744. This arrangement uses a temperature sensor in the adsorption filter of the tank-venting apparatus. The temperature of adsorbent material increases with an adsorption of fuel whereas, the temperature drops for a desorption of fuel. These temperature changes can be determined with the aid of the temperature sensor. Temperature changes are, however, also caused by fuel vapors which have a temperature different than the adsorbent material. For this reason, a definite decision cannot always be made as to whether the particular temperature change is caused by proper operation of the adsorption filter or by a disturbance effect. However, to nonetheless arrive at a reliable statement, U.S. Pat. No. 4,962,744 teaches that measurements are to take place only during a tanking operation and directly after such a tanking operation. Then, for a properly operating tank-venting apparatus, especially intense and therefore especially pronounced temperature changes can be expected.
U.S. Pat. No. 5,193,512 teaches that the pressure in the tank-venting apparatus is to be monitored for checking the operability of a tank-venting apparatus not only in direct association with tanking operations. As soon as a tank-venting valve is opened, the underpressure in the apparatus should increase whereas, the pressure should again decrease when this valve is closed. This patent discloses a tank-venting apparatus having a controllable shutoff valve in the venting line of the adsorption filter in order to obtain a relatively large measuring effect. This shutoff valve is closed for the tank-venting diagnosis so that an underpressure effect is obtained as rapidly as possible and which is clearly recognizable; whereas, the shutoff valve is opened when the adsorption filter is to be regenerated.
The shutoff valve described in U.S. Pat. No. 5,193,512 advantageously includes an underpressure protective valve so that an underpressure cannot build up which is too great and which could lead to damage of the tank-venting apparatus in the event that there is a defect of the shutoff valve.
The above-cited detail solution indicates that operational safety is pertinent in the arrangements and methods for conducting a tank-venting diagnosis.